In a conventional optical system utilizing light emitting diodes (LED or LEDs) as the light source and only a single lens as the optic, the LEDs are aligned so that the central axis of light from the LEDs is pointed at the center of the field to be illuminated and the lens is perpendicular to the axis orientation. Formed on the side of the lens facing the LEDs is a Fresnel refractor and formed on the side of the lens facing the field to be illuminated is a series of pillow-type lens elements. The LED light is directed by the Fresnel element in both the horizontal and vertical directions into a collimated beam. The external pillow lens elements then direct the collimated beam into the required vertical and horizontal angular light distributions. The disadvantage of this design is that such a lens may not be perpendicular to the optical axis of the test pattern. This may be due to actual construction or because it is inconvenient to position the lens vertically in the preferred vehicle design. Typically the windows are sloped, thereby requiring a slopped lens face. The circular Fresnel lens collimates light along the axis of the lens tilt rather than the optical axis, which makes the system incapable of meeting light distribution. In the past, if it was desired that a lens tilt not be perpendicular to the optical axis, an additional inner lens piece with the Fresnel or the Fresnel and the pillows that was perpendicular to the optical axis was used. This inner lens increased the cost and reduced the amount of available light.
Center high mount stop lamps (CHMSLs) that used incandescent lamps provided sufficient excess light so that losing some light still allowed the light output to meet legal specifications. However, to use LEDs there is a much greater need to be efficient. At the same time there is a need for standardized lamps systems usable in a variety of vehicles with differing window slopes. LED CHMSLs have been made with lenses provided on the inside surface with one or more circular Fresnel lens area that would receive and collimate the light respectively fro a corresponding LED. The collimated light passed through the lens to the exterior surface where it encountered square, pillow type lens elements the spread the light vertically and horizontally. This lens had to be aligned so that the central, i.e., axial radiation from the LED went through the center of the corresponding Fresnel lens portion and was parallel to the normal axis of the lens. If the lens were positioned so that the LED axis was at an angle to the lens normal, for example, where a user wants the lens to have a different face angle to fit against a window, then the lens would direct light substantially along the axis of tilt. For an LED lamp system there may be too little light to start with so the misdirection amounted to an intolerable light loss from the obligated legal requirements.